Chemists have great skill when it comes to knowing what chemicals will be given off in reactions and how these will interact with already present chemicals and so on. For instance if a lot of a halogenoalkane like methane chloride (CH3Cl) reacts with ammonia (NH3) then quite simply the whole CH3 that was attached to the Cl previously replaces one H on the NH3. This leaves hydrochloric acid (H+Cl from the removed parts) and methylamine (CH3NH2). But since there is still CH3Cl present it can react and replace another hydrogen on the nitrogen. This happens again and again until you are left with:
So the main puzzle that gets created for biologists is how do they make a useful reaction occur where they want it. The chemical they want delivered is hydrogen sulphide (H2S) as in small quantities it can help reduce oxidation in cells. Hydrogen sulphide is the chemical that makes rotten eggs spell so bad and generally when the human body has evolved to be repelled by something you really don’t want to go near it. Anyone who has encountered H2S in a pure form will know the safety procedures necessary and so to get it into the body safely is a challenge. To perform this feat the scientists noticed that when human cells are under oxidised stress, species like hydrogen peroxide (H2O2) are formed. The solution is to build a specific biological molecule that contains carbonyl sulphide (COS, which rapidly is converted into H2S by the body) and then releases the COS when a trigger molecule, like the hydrogen peroxide, reacts with it. It is still unclear how well this method will work in vivo practice but in laboratories results already seem promising.